The investigations with which the invention is concerned involve lowering a probe down the hole. The probe contains sensors which detect and identify the fluids encountered.
It is known that the difference between air and liquid may be detected by means of an optical sensor. A beam of light is made to shine through an interface between the fluid and a transparent material such as glass. If the fluid is air, the beam is refracted in one mode; if the fluid is water or another liquid, such as gasoline, the beam is refracted in another mode. A light sensor, such as a phototransistor, is used to signal the different modes of refraction, and thus to signal the level at which the liquid is encountered.
The optical sensor does not determine the type of liquid, however. Although both water and gasoline have refractive indexes which differ substantially from that of air, they do not differ very much from each other. It is impractical to use refraction as the agency for distinguishing water from gasoline. (Although gasoline is a common contaminant encountered in ground water, the technology is applicable also to other hydrocarbon contaminants: the generic term "oil" is used hereinafter.)
It is known that the difference between oil and water may be detected by measuring the electrical conductivity of the liquid. The resistivity of oil is very high: in fact oil is, in practical terms, indistinguishable from air on the basis of its resistivity. The resistivity of water, on the other hand, is comparatively low.
The down-hole probe therefore contains an optical sensor and a conductivity sensor. The optical sensor contains a light emitter (LED) which generally remains ON while the probe is down the hole, and a light sensor, which detects whether or not the light beam from the light has been refracted. The conductivity sensor comprises a pair of electrodes which are immersed in the liquid, and the resistivity of the liquid is measured by passing a current therebetween.
The probe is attached to a suspension tape, by means of which the probe is lowered down the hole. The tape is marked with a scale to indicate the lowered depth. It is an advantage for the tape to also include within its structure the electrical leads necessary for the operation of the sensors, rather than separate loose leads. Hitherto, to operate the two sensors it has been necessary to provide at least three electrical leads running between the probe and the surface.
The invention provides an apparatus which requires the use of only two leads, yet which is aimed at providing reliable operation of the two sensors.